1. Field of the Invention
The present invention relates to inner focus lenses, interchangeable lens apparatuses, and camera systems. Specifically, the present invention relates to: inner focus lenses suitable for an interchangeable lens apparatus attachable to a digital single-lens reflex camera or a single-lens reflex camera using silver salt films, a digital still camera, a camcorder, and the like; and interchangeable lens apparatuses and camera systems employing these inner focus lenses.
2. Description of the Background Art
In association with increases in the number of pixels of solid-state image sensors in recent years, imaging optical systems employed for them are requested to have higher performance as well as to be a bright lens having a small F-number. Further, strong demands are present for higher-speed focusing and for lenses in which image vibration at the time of focusing is reduced. Moreover, strong demands are present also for size reduction and cost reduction in the optical systems, and hence such optical systems are requested to be constructed from a small number of lenses.
In order that such demands should be satisfied, a lens system disclosed in Japanese Laid-Open Patent Publication No. S50-138823 has been proposed which is a so-called Gauss type lens system, in order from the object side to the image side, comprising a front lens unit having positive refractive power and a rear lens unit having positive refractive power and in which focusing is achieved by a positive lens unit included in the rear lens unit. In such a Gauss type lens system, a large aperture is implemented and yet, compensation of aberration is achieved easily.
However, in the lens system disclosed in Japanese Laid-Open Patent Publication No. S50-138823, focusing is performed by the positive lens unit included in the rear lens unit. This causes an increase in the amount of movement of the positive lens unit at the time of focusing, and hence high-speed focusing demanded in recent years is not realized.